1. Technical Field of the Invention
The present invention relates to a light condensing filter in which plural prisms are formed on a substrate and light from a light source is condensed by the plural prisms.
2. Description of the Related Art
A liquid crystal display device varies transmissivity or reflectivity to light by using electro-optical effects such as optical anisotropy, orientation, fluidity and dielectric anisotropy of liquid crystal molecules to thereby display images. This liquid crystal display device is designed to be thin and light and also it has low power consumption, so that it has been broadly used for personal computers, television sets, portable terminals, etc.
The liquid crystal display device comprises a light source for back light (cold cathode fluorescent lamp (CCFL)), a light guide plate for guiding light from the light source, a diffusion filter for diffusing light, a prism sheet for condensing the light from the light guide plate so that the light is incident to a liquid crystal layer, a liquid crystal display panel having the liquid crystal layer, etc. which are successively laminated. In the liquid crystal display device, the light from the light source is guided from the light guide plate to the diffusion film, and the light which is uniformly diffused by the diffusion film is condensed by the prism sheet and incident to the liquid crystal layer. With the prism sheet, the display brightness of the liquid crystal display device can be enhanced.
There is known a related art prism sheet in which plural prisms are regularly arranged on the surface and back surface of a flat substrate and the arrangement direction of prisms arranged on the surface is perpendicular to the arrangement direction of prisms arranged on the back surface (see JP-A-8-262206 and JP-A-11-109134).
Furthermore, there is also known a prism sheet in which plural prisms are irregularly arranged on the surface of a flat substrate (see Products catalog of Brightness-enhanced film “BEF-III series” produced by Sumitomo 3M Ltd., which is hereinafter referred to as non-patent document 1).
FIG. 7 is a perspective view showing the construction of the prism sheet described in the non-patent document 1. In the prism sheet of FIG. 7, plural prisms 1 having the same top angle (the angle of the top portion) are formed on the surface of the flat substrate 2 so as to be continuous in the X direction. In FIG. 7, each boundary line of the plural prisms 1 is represented by a broken line vertically-extending from a valley portion 4 between the respective prisms to the surface. The height of the top portion 3 of each prism from the surface and the height of the valley portion 4 between the respective prisms from the surface are dispersed, so that the arrangement pitch of the plural prisms 1 (the distance between the midpoints in the width direction of two adjacent prisms in an X-direction) is dispersed and the plural prisms 1 are irregularly arranged in the X-direction.
According to the prism sheet of FIG. 7, since the arrangement of the plural prisms 1 is irregular, interference with the arrangement of display pixels, color filters or the like of a liquid crystal display device having this prism sheet mounted therein can be prevented, and thus moiré can be suppressed. The display brightness of the liquid crystal display device can be enhanced by using two prism sheets of FIG. 7. In this case, the two prism sheets are disposed between the light guide plate and the liquid crystal layer in the liquid crystal display device so that the arrangement directions of the plural prisms of the prism sheets are perpendicular to each other.
According to the prism sheets described in JP-A-8-262206 and JP-A-11-109134, the plural prisms are arranged on the surface and the back surface, and thus the light condensing effect is enhanced. However, the prisms formed on the surface and the back surface are regularly arranged, and thus moiré occurs in the liquid crystal display device having this prism sheet mounted therein.
According to the prism sheet described in the non-patent document 1, the heights of the top portions of the plural prisms 1 from the surface are not set to be equal to one another. Accordingly, when optical film such as a diffusion plate or the like or a liquid crystal layer is disposed on the plural prisms 1 of this prism sheet, flatness of the optical film or the liquid crystal layer is lost, and thus there is a problem that unevenness of light occurs or the like.
The present invention has been implemented in view of the foregoing situation.